This invention relates to a method and means for controlling the flux density in the core of an inductor such as a transformer or choke, in order to enhance the A.C. performance thereof in the presence of a direct or low frequency current circulating therein, and, if desired, to enable the magnitude of such a direct or low frequency current to be determined without any electrical contact being made to the circuit in which these currents flow.
It is well known that the strength of magnetic fields may be determined in a number of ways, for example, using devices which operate on the "Hall Effect" principle. It is also well known that direct current may be passed through control windings on, for example, magnetic amplifier transformers or inductors, enabling the magnetic circuit of such transformers or inductors to be saturated at will.
Many transformers and inductors used for example in the field of telecommunications, have to operate in the presence of relatively large direct circulating currents which result in high magnetic flux density levels within the core of the device. All magnetic core materials, including ferrites have a limiting flux density above which they cease to function satisfactory. This level is referred to as the "Saturation Flux Density" of the material which is a function of the material and its magnetic history. The magnetic flux density in the core is a function of the magnetising force (broadly the vector sum of the ampere turns in all windings), and the permeability of the core assembly. Although the alternating current performance of the transformer or inductor is enhanced by a highly permeable core assembly, effective permeability has frequently to be limited by the introduction of an air gap, or the use of magnetic material of inferior permeability to avoid exceeding the maximum saturation flux density of the material. The cross-sectional area of the core has also to be increased by a substantial factor to cater for the direct current flowing, beyond the area which would otherwise be needed to cater for the alternating currents of interest. This increases the size and cost of the transformer or inductor very considerably and generally degrades A.C. performance. Physical and economic limitation therefore often impose a technically undersirable design compromise. Similar considerations apply in the case of power supply filter chokes.